There is a speaker unit which outputs sound amplified by an amplifier, and the speaker unit is assembled in a housing, thereby constituting an acoustic device. For example, there is a speaker unit configured so that a vibration plate is vibrated by a magnetic circuit having a magnet, a yoke, and a coil.
In addition to the magnetic circuit, such a speaker unit is provided with a bobbin which is varied in an axial direction by the magnetic circuit, a vibration plate vibrated in accordance with the variation of the bobbin, and a frame which holds the vibration plate and the magnetic circuit.
In the speaker unit, in order to ensure a good output state of sound, it is necessary to suppress deterioration of the sound quality or to improve directivity characteristics in a predetermined direction, and for example, there is a speaker unit in which a vibration plate is eccentric to a center axis of a bobbin in order to ensure such a good output state of sound (for example, refer to Patent Document 1).
In the speaker unit described in Patent Document 1, a main vibration plate and an auxiliary vibration plate are provided as a vibration plate, and the auxiliary vibration plate is attached to an inner peripheral portion of the main vibration plate. The main vibration plate and the auxiliary vibration plate are formed in a shape which is eccentric to the center axis of the bobbin at predetermined distances, respectively.
As described above, in the speaker unit described in Patent Document 1, since the main vibration plate and the auxiliary vibration plate are eccentric to the center axis of the bobbin, it is possible to suppress the occurrence of the resonance and broaden the band, thereby securing a good output state of sound.